Unit 3 States and changes of states of matter
Key unit competence: Relate the properties of matter to daily life physical and chemical phenomena.
Key ideas
- Matter is anything that has mass and occupies space. It exists in three main states: solid, liquid and gas.
- Kinetic theory: matter is made of tiny particles that are always moving. In solids they vibrate in fixed positions and are close together; in liquids they are close but move past one another; in gases they are far apart and move fast in all directions.
- Changes of state: melting (solid → liquid), freezing (liquid → solid), evaporation/boiling (liquid → gas), condensation (gas → liquid), sublimation (solid → gas directly, e.g. iodine, dry ice) and deposition (gas → solid).
- Heating gives particles more kinetic energy. During melting and boiling, temperature stays constant because the energy is used to overcome forces between particles.
- A physical change forms no new substance and is usually reversible (melting ice, dissolving sugar). A chemical change forms new substances and is usually irreversible (burning paper, rusting, cooking an egg).
- Diffusion is the spreading of particles from high to low concentration; it is fastest in gases. Evaporation happens at any temperature at the surface; boiling happens throughout the liquid at the boiling point.
Common misconceptions
- "Particles of a solid do not move at all." No: They vibrate about fixed positions.
- "Temperature keeps rising while water boils." No: It stays at 100 °C (at sea level) until all the water has boiled.
- "Dissolving sugar is a chemical change because the sugar disappears." No: The sugar is still there (the water tastes sweet); it is a physical change.
Worked examples
Example 1. Use the kinetic theory to explain why a solid has a fixed shape but a gas does not.
Solution
In a solid, particles are close together and held by strong forces in fixed positions; they only vibrate, so the shape is fixed.
In a gas, particles are far apart, forces are very weak and particles move freely in all directions, so a gas fills any container.
Example 2. Classify each change as physical or chemical: (a) melting candle wax, (b) burning a candle, (c) rusting nail.
Solution
(a) Physical: no new substance, wax can solidify again.
(b) Chemical: new substances (carbon dioxide and water) form.
(c) Chemical: new substance, rust (hydrated iron(III) oxide), forms.
Practice questions
Define matter.
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Anything that has mass and occupies space.
Common mistake: Saying 'anything you can see', since air is matter but invisible.
Name the change of state from gas to liquid.
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Condensation.
Common mistake: Writing 'evaporation', which is liquid to gas.
Give two differences between a physical and a chemical change.
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Physical: no new substance, usually reversible. Chemical: new substance formed, usually irreversible, often with energy change.
Common mistake: Saying all physical changes involve heat.
Why does a liquid flow?
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Its particles can move past one another.
Common mistake: Saying its particles are far apart.
Explain why temperature stays constant while ice is melting.
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The heat absorbed is used to overcome forces between particles (latent heat), not to raise the temperature.
Common mistake: Saying the heater stopped working.
State two factors that increase the rate of evaporation.
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Higher temperature, larger surface area, wind (moving air), low humidity (any two).
Common mistake: Saying 'more water' increases rate.
Why does the smell of perfume spread across a room?
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Perfume particles diffuse through the air from high to low concentration.
Common mistake: Saying the air carries it only because of wind.
Classify: (a) souring of milk, (b) dissolving sugar.
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(a) Chemical; (b) physical.
Common mistake: Calling dissolving chemical because the sugar 'disappears'.
A substance has melting point 1083 °C. What is its state at room temperature? Explain.
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Solid, because room temperature (about 25 °C) is below its melting point.
Common mistake: Comparing with boiling point instead.
Explain why gases exert pressure on the walls of a container.
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Moving gas particles collide with the walls and exert a force on them.
Common mistake: Saying gases are heavy.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Evaporation differs from boiling because evaporation: a. Solid 2. Which change is physical? b. Dissolving salt in water 3. Which is a sign of a chemical change? c. A new substance with different properties forms 4. Which state of matter has a fixed shape and a fixed volume? d. happens at the surface at any temperature Show answer
1 → d (happens at the surface at any temperature) · 2 → b (Dissolving salt in water) · 3 → c (A new substance with different properties forms) · 4 → a (Solid)
- Boiling occurs throughout the liquid at the boiling point; evaporation is surface-only and at any temperature.
- The salt can be recovered by evaporation; no new substance forms.
- Chemical changes produce new substances, often with gas, colour change or heat.
- Particles in a solid are held in fixed positions, so both shape and volume are fixed.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
can move past one another · diffusion · do not move · condensation- The smell of food cooking reaches you in another room by .
- Water vapour turning into water droplets on a cold window is called .
- A liquid takes the shape of its container because its particles .
Show answer
1: diffusion · 2: condensation · 3: can move past one another
- Gas particles of the smell spread out through the air.
- Gas changes to liquid when it is cooled: condensation.
- Liquid particles are close together but can slide over each other.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Which substance sublimes when heated? a. They vibrate faster 2. Which gas diffuses faster: ammonia (NH3, M = 17) or hydrogen chloride (HCl, M = 36.5)? b. Iodine 3. What happens to particles when a solid is heated? c. Liquid 4. A substance melts at −7 °C and boils at 59 °C. What is its state at 25 °C? d. Ammonia, because its particles are lighter Show answer
1 → b (Iodine) · 2 → d (Ammonia, because its particles are lighter) · 3 → a (They vibrate faster) · 4 → c (Liquid)
- Iodine crystals turn directly into purple vapour.
- Lighter particles move faster at the same temperature, so ammonia diffuses faster.
- Heating gives particles more kinetic energy, so they vibrate more.
- 25 °C is above the melting point and below the boiling point, so it is liquid.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
boiling point · Gas · freezing · melting- The change from liquid to solid is called .
- The temperature at which a pure liquid boils at normal pressure is its .
- In which state are the particles furthest apart? →
Show answer
1: freezing · 2: boiling point · 3: Gas
- Freezing is when a liquid loses energy and becomes solid.
- Pure water boils at 100 °C at standard pressure.
- Gas particles are far apart and move freely.